Manufacturing method of cathode active material, and cathode active material for lithium secondary battery manufactured thereby

ABSTRACT

Provided are a method of preparing a cathode active material including coating a surface of a lithium transition metal oxide with a lithium boron oxide by dry mixing the lithium transition metal oxide and a boron-containing compound and performing a heat treatment, and a cathode active material prepared thereby. 
     A method of preparing a cathode active material according to an embodiment of the present invention may easily transform lithium impurities present in a lithium transition metal oxide into a structurally stable lithium boron oxide by performing a heat treatment near the melting point of a boron-containing compound. 
     Also, a coating layer may be formed in which the lithium boron oxide is uniformly coated in an amount proportional to the used amount of the boron-containing compound even at a low heat treatment temperature.

TECHNICAL FIELD

The present invention relates to a manufacturing method of a cathodeactive material, and cathode active material for lithium secondarybattery manufactured thereby.

BACKGROUND ART

Lithium secondary batteries have been widely used as power sources ofportable devices after they have emerged as small, lightweight, andhigh-capacity batteries since 1991. Recently, in line with the rapiddevelopment of electronics, communications, and computer industries,camcorders, mobile phones, and notebook PCs have appeared and undergonecontinuous and remarkable development. Accordingly, the demand forlithium secondary batteries as a power source for driving these portableelectronic information and communication devices has increased day byday.

Lithium secondary batteries have limitations in that their lifetimerapidly decreases as charge and discharge are repeated. In particular,the above limitations are more severe at high temperature. The reasonfor this is due to a phenomenon that occurs when an electrolyte isdecomposed or an active material is degraded due to moisture in thebattery or other effects, and the internal resistance of the batteryincreases.

In order to address the above limitations, a technique of coating thesurface of a cathode active material with an oxide of metal, such asmagnesium (Mg), aluminum (Al), cobalt (Co), potassium (K), sodium (Na),and calcium (Ca), by a heat treatment has developed. Also, research toimprove energy density and high-rate characteristics by adding TiO₂ to aLiCoO₂ active material has been conducted.

However, limitations, such as lifetime degradation or gas generation dueto the decomposition of the electrolyte during charge and discharge,have not been fully resolved yet.

In the case that impurities are present in the surface of a cathodeactive material during a process of fabricating an electrode of alithium secondary battery, the impurities may not only affect aging in astep of preparing an electrode slurry during the process of fabricatingan electrode of a lithium secondary battery, but may also cause aswelling phenomenon in the lithium secondary battery by reacting with anelectrolyte solution that is injected into the lithium secondarybattery.

In order to address the above limitations, a method of coating thesurface of a cathode active material with H₃BO₃ has been developed.

Examples of the above method may include a method of coating the surfaceof a cathode active material by mixing the cathode active material withH₃BO₃ by shaking several times using a shaker. However, in this case,H₃BO₃ particles may agglomerate on the surface of the cathode activematerial.

As another example, there is a method of coating a cathode activematerial by mixing the cathode active material and H₃BO₃ usingmechanical compositing equipment, for example, a Nobilta™ device. Inthis case, since an amount of a coating layer included in the cathodeactive material is not increased when H₃BO₃ is added in a predeterminedamount or more, there may be limitations in the reaction process.

Therefore, there is an urgent need to develop a method of preparing acathode active material which may improve the performance of a lithiumsecondary battery while addressing the above limitations.

PRIOR ART DOCUMENTS Patent Document

Japanese Patent Application Laid-Open Publication No. 2009-152214

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a method of preparing acathode active material which may transform lithium impurities presentin a lithium transition metal oxide into a structurally stable lithiumboron oxide by performing a heat treatment using a boron-containingcompound.

Another aspect of the present invention provides a cathode activematerial, which includes a coating layer including a lithium boron oxideon the surface of the lithium transition metal oxide, by the method ofpreparing a cathode active material.

Another aspect of the present invention provides a cathode and a lithiumsecondary battery including the cathode active material.

Technical Solution

According to an aspect of the present invention, there is provided amethod of preparing a cathode active material including coating asurface of a lithium transition metal oxide with a lithium boron oxideby dry mixing the lithium transition metal oxide and a boron-containingcompound and performing a heat treatment.

According to another aspect of the present invention, there is provideda cathode active material including: a lithium transition metal oxide;and a coating layer including a lithium boron oxide on a surface of thelithium transition metal oxide.

According to another aspect of the present invention, there is provideda cathode including the cathode active material.

According to another aspect of the present invention, there is provideda lithium secondary battery including the cathode.

Advantageous Effects

A method of preparing a cathode active material according to anembodiment of the present invention may easily transform lithiumimpurities present in a lithium transition metal oxide into astructurally stable lithium boron oxide by performing a heat treatmentnear the melting point of a boron-containing compound.

Also, a coating layer may be formed in which the lithium boron oxide isuniformly coated on the surface of the lithium transition metal oxide inan amount proportional to the used amount of the boron-containingcompound even at a low heat treatment temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings attached to the specification illustratepreferred examples of the present invention by example, and serve toenable technical concepts of the present invention to be furtherunderstood together with detailed description of the invention givenbelow, and therefore the present invention should not be interpretedonly with matters in such drawings.

FIG. 1 is a schematic diagram illustrating a method of preparing acathode active material according to an embodiment of the presentinvention;

FIG. 2 is a graph illustrating the results of pH titration toinvestigate the amounts of lithium impurities of cathode activematerials prepared in Example 1 of the present invention and ComparativeExamples 1 and 2;

FIG. 3 is a graph illustrating the results of measuring capacitycharacteristics after high-temperature storage (60° C.) of lithiumsecondary batteries of Example 5 and Comparative Example 4 according toExperimental Example 3; and

FIG. 4 is a graph illustrating the results of measuring high-temperature(45° C.) cycle characteristics of lithium secondary batteries of Example5 and Comparative Example 4 according to Experimental Example 4.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail toallow for a clearer understanding of the present invention.

It will be understood that words or terms used in the specification andclaims shall not be interpreted as the meaning defined in commonly useddictionaries. It will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

As illustrated in FIG. 1, a method of preparing a cathode activematerial according to an embodiment of the present invention may includecoating a surface of a lithium transition metal oxide with a lithiumboron oxide by dry mixing the lithium transition metal oxide and aboron-containing compound and performing a heat treatment.

The method of preparing a cathode active material according to theembodiment of the present invention may easily transform lithiumimpurities present in a lithium transition metal oxide into astructurally stable lithium boron oxide by dry mixing the lithiumtransition metal oxide and a boron-containing compound and performing aheat treatment, in particular, near a melting point of theboron-containing compound. Also, a coating layer may be formed in whichthe lithium boron oxide is uniformly coated on the surface of thelithium transition metal oxide in an amount proportional to the usedamount of the boron-containing compound even at a low heat treatmenttemperature.

In the method of preparing a cathode active material according to theembodiment of the present invention, the boron-containing compound maybe any one selected from the group consisting of H₃BO₃, B₂O₃,C₆H₅B(OH)₂, (C₆H₅O)₃B, [CH₃(CH₂)₃O]₃B, C₁₃H₁₉BO₃, C₃H₉B₃O₆, and(C₃H₇O)₃B, or a mixture of two or more thereof.

In general, a method of forming a coating layer on the surface of thelithium transition metal oxide may include a dry mixing method and a wetmixing method. In the case that the wet mixing method is used, a moreuniform coating layer formed on the surface of the lithium transitionmetal oxide may be obtained. However, with respect to the wet mixingmethod, the boron-containing compound must be used in a state of anaqueous solution, and in this case, there is a possibility that damageto the lithium transition metal oxide occurs due to the aqueoussolution.

Thus, according to the method of preparing a cathode active materialaccording to the embodiment of the present invention, since theboron-containing compound is dry mixed with the lithium transition metaloxide and a heat treatment is performed near the melting point of theboron-containing compound to melt and flow the boron-containingcompound, the advantage of the wet mixing method, i.e., a uniformcoating layer, may be realized without causing the damage to the lithiumtransition metal oxide, i.e., the limitation of the wet mixing method.

Specifically, the boron-containing compound, for example, H₃BO₃, beginsto be melted while being softened at low temperature, for example, in atemperature range of about 130° C. to about 160° C.

Since the boron-containing compound may react with at least a portion oflithium impurities present in the lithium transition metal oxide whilebeing melted and flowing by the heat treatment, the boron-containingcompound may be easily transformed into a lithium boron oxide to becoated on the surface of the lithium transition metal oxide. Thus, thelithium impurities present in the lithium transition metal oxide may bereduced by the transformation of the lithium impurities into the lithiumboron oxide.

According to the method of preparing a cathode active material accordingto the embodiment of the present invention, the heat treatment isperformed in a temperature range of 130° C. to 300° C., near the meltingpoint of the boron-containing compound, and may be performed in atemperature range of 130° C. to 200° C., for example, for 3 hours to 10hours.

In the case that the heat treatment temperature is less than 130° C.,since the boron-containing compound is not sufficiently melted, theboron-containing compound may remain as it is or may not form a uniformcoating layer even if it is transformed into the lithium boron oxide. Inthe case in which the heat treatment temperature is greater than 300°C., since the reaction may be excessively fast due to the hightemperature, a uniform coating layer may not be formed on the surface ofthe lithium transition metal oxide.

According to the method of preparing a cathode active material accordingto the embodiment of the present invention, since the heat treatment isperformed at a specific temperature, a coating layer may be formed inwhich the lithium boron oxide is uniformly coated on the surface of thelithium transition metal oxide in an amount proportional to the usedamount of the boron-containing compound.

In the preparation method according to the embodiment of the presentinvention, the dry mixing may be performed by a mortar grinder mixingmethod and a mechanical milling method. For example, it may be desirableto use the mechanical milling method to form a uniform coating layer.

Specifically, in the mortar grinder mixing method, lithium transitionmetal oxide and boron-containing compound are uniformly mixed using amortar, and a heat treatment may then be performed in the above heattreatment temperature range.

Also, the mechanical milling method, for example, may mix lithiumtransition metal oxide and boron-containing compound by mechanicalattrition using a roll mill, ball mill, high energy ball mill, planetarymill, stirred ball mill, vibrating mill, or jet mill, and for example,compressive stress may be mechanically applied by rotating at a speed of100 rpm to 1,500 rpm.

In the case that the mechanical milling method is used, the lithiumtransition metal oxide and boron-containing compound are mixed by themechanical milling method, and then the mixture may be heat treated inthe above temperature range or mixing and heat treatment may besimultaneously performed in the above milling device. According to anembodiment of the present invention, the mechanical milling method,instead of the mortar grinder mixing method, may be used to form auniform coating layer.

In the method of preparing a cathode active material according to theembodiment of the present invention, an amount of the boron-containingcompound used is in a range of 0.05 wt % to 1 wt % and may be in a rangeof 0.1 wt % to 0.8 wt % based on a total weight of the lithiumtransition metal oxide.

According to an embodiment of the present invention, an amount ofelemental boron (B) included in the coating layer of the cathode activematerial may be increased as the amount of the boron-containing compoundused is increased within the above range.

Also, according to an embodiment of the present invention, a portion ofthe elemental B of the lithium boron oxide may be doped into the lithiumtransition metal oxide by the heat treatment, and the amount of the Bmay have a concentration gradient gradually decreasing from the surfaceof the lithium transition metal oxide to the inside thereof.

Furthermore, the present invention provides a cathode active materialwhich includes a lithium transition metal oxide; and a coating layerincluding a lithium boron oxide on a surface of the lithium transitionmetal oxide.

The coating layer may include elemental B in an amount of 100 ppm to2,000 ppm, for example, 250 ppm to 1,100 ppm.

Also, in the cathode active material according to the embodiment of thepresent invention, the lithium boron oxide included in the coating layermay be included in an amount of 0.05 wt % to 1 wt %, for example, 0.1 wt% to 0.8 wt % based on a total weight of the cathode active material.

In the case that the amount of the lithium boron oxide is less than 0.05wt %, since a thickness of the coating layer formed on the surface ofthe lithium transition metal oxide may be decreased, an effect ofsuppressing side reactions between electrolytes during charge anddischarge may be insignificant. In the case in which the amount of thelithium boron oxide is greater than 1 wt %, since the thickness of thecoating layer may be increased due to the excessive amount of thelithium boron oxide, electrochemical properties of a lithium secondarybattery may be reduced due to the resulting increase in resistance.

The lithium boron oxide may be LiBO₂, Li₂B₄O₇, or a mixture thereof.

Also, the thickness of the coating layer may be in a range of 10 nm to1,000 nm.

In the cathode active material according to the embodiment of thepresent invention, a typically used lithium transition metal oxide maybe used as the lithium transition metal oxide, and examples of thelithium transition metal oxide may be any one selected from the groupconsisting of a lithium-cobalt-based oxide, a lithium-manganese-basedoxide, a lithium-nickel-manganese-based oxide, alithium-manganese-cobalt-based oxide, and alithium-nickel-manganese-cobalt-based oxide, or a mixture of two or morethereof. In particular, a layered-structure lithium transition mealoxide having high capacity characteristics may be used and may berepresented by Chemical Formula 1 below:

Li_(1+a)[Ni_(x)Mn_(y)Co_(z)M_(v)]O_(2-c)A_(c)  <Chemical Formula 1>

where M is any one selected from the group consisting of aluminum (Al),zirconium (Zr), zinc (Zn), titanium (Ti), magnesium (Mg), gallium (Ga),and indium (In), or two or more elements thereof; A is at least oneselected from the group consisting of phosphorus (P), fluorine (F),sulfur (S), and nitrogen (N), and 0≦x≦1.0, 0≦y<0.6, 0≦z<0.6, 0≦v≦0.1,0≦a<0.3, 0≦c≦0.2, and a+x+y+z+v=1.

The lithium impurities, which may be present in the surface of thelithium transition metal oxide, may include at least one of LiOH andLi₂CO₃.

For example, the lithium impurities having the following ChemicalFormula 2 may be included in the lithium transition metal oxide:

(1-s-t)[Li(Li_(a)Mn_((1-a-x-z))Ni_(x)Co_(z))O₂ ].s[Li₂CO₃].t[LiOH]  <Chemical Formula 2>

where 0≦a<0.3, 0≦x<0.9, 0≦z<0.6, 0<s<0.05, and 0<t<0.05.

The lithium impurities present in the lithium transition metal oxide maybe included in an amount of 0.1 wt % to 0.6 wt % based on the totalweight of the lithium transition metal oxide.

According to an embodiment of the present invention, since at least aportion of the lithium impurities in the lithium transition metal oxidereacts to be transformed into the lithium boron oxide, the amount of thelithium impurities may be decreased by about 30% to about 70%, forexample, about 40% to about 70% in comparison to the amount of thelithium impurities before the lithium impurities are transformed intothe lithium boron oxide.

Specifically, the lithium impurities present in the lithium transitionmetal oxide according to the present invention may be included in anamount of less than 0.3 wt % based on the total weight of the cathodeactive material. The reason for this is that since the lithiumimpurities, such as LiOH or Li₂CO₃, may have high reactivity with theelectrolyte, a swelling phenomenon may excessively occur in the casethat the amount of the lithium impurities present in the lithiumtransition metal oxide is 0.3 wt % or more.

Also, according to an embodiment of the present invention, a traceamount of the elemental B of the lithium boron oxide may be furtherincluded by doping into the lithium transition metal oxide.

In the case that the lithium transition metal oxide is doped with aportion of the elemental B of the lithium boron oxide, the lithiumtransition metal oxide, for example, is represented by Chemical Formula3 below.

Li_(1+a)[Ni_(x)Mn_(y)Co_(z)B_(w)M_(v)]O_(2-c)A_(c)  <Chemical Formula 3>

where M is any one selected from the group consisting of Al, Zr, Zn, Ti,Mg, Ga, and In, or two or more elements thereof; A is at least oneselected from the group consisting of P, F, S, and N, and 0≦x≦1.0,0≦y<0.6, 0≦z<0.6, 0≦v≦0.1, 0≦a<0.3, 0≦c≦0.2, a+x+y+z+v=1, and 00.1.

In this case, w may have a concentration gradient gradually decreasingfrom the surface of the lithium transition metal oxide to the insidethereof.

The present invention provides a cathode including the cathode activematerial.

Also, the present invention provides a cathode including the cathodeactive material.

The cathode may be prepared according to a typical method known in theart. For example, a binder, a conductive agent, and a dispersant, ifnecessary, as well as a solvent are mixed with a cathode active materialand stirred to prepare a slurry, and a metal current collector is thencoated with the slurry and pressed. Thereafter, the cathode may beprepared by drying the metal current collector.

Any metal may be used as the metal current collector so long as it is ametal having high conductivity as well as no reactivity in a voltagerange of the battery to which the slurry of the cathode active materialmay be easily adhered. Non-limiting examples of the cathode collectormay include aluminum, nickel, or a foil prepared by a combinationthereof.

An organic solvent, such as N-methylpyrrolidone (NMP), dimethylformamide(DMF), acetone, and dimethylacetamide, or water may be used as thesolvent for forming the cathode. These solvents may be used alone or ina mixture of two or more thereof. An amount of the solvent used may besufficient if the solvent may dissolve and disperse the cathode activematerial, the binder, and the conductive agent in consideration of acoating thickness of the slurry and manufacturing yield.

Various types of binder polymers, such as a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol,carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose,regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, polyacrylic acid, anethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, astyrene-butadiene rubber (SBR), a fluorine rubber, poly(acrylic acid),polymers in which hydrogens thereof are substituted with Li, sodium(Na), or Ca, or various copolymers, may be used as the binder.

Any conductive agent may be used without particular limitation so longas it has suitable conductivity without causing adverse chemical changesin the batteries. For example, the conductive agent may include aconductive material such as: graphite such as natural graphite andartificial graphite; carbon black such as carbon black, acetylene black,Ketjen black, channel black, furnace black, lamp black, and thermalblack; conductive fibers such as carbon fibers and metal fibers;conductive tubes such as carbon nanotubes; metal powder such asfluorocarbon powder, aluminum powder, and nickel powder; conductivewhiskers such as zinc oxide whiskers and potassium titanate whiskers;conductive metal oxide such as titanium oxide; or polyphenylenederivatives.

Also, the present invention provides a lithium secondary batteryincluding the cathode, an anode, and a separator disposed between thecathode and the anode.

A carbon material, lithium metal, silicon, or tin, which may intercalateand deintercalate lithium ions, may be typically used as an anode activematerial that is used in the anode according to an embodiment of thepresent invention.

Furthermore, the anode collector is generally fabricated to have athickness of 3 μm to 500 μm. The anode collector is not particularlylimited so long as it has conductivity without causing adverse chemicalchanges in the batteries. The anode collector may be formed of, forexample, copper, stainless steel, aluminum, nickel, titanium, bakedcarbon, copper or stainless steel that is surface-treated with one ofcarbon, nickel, titanium, silver, or the like, an aluminum-cadmiumalloy, or the like. Similar to the cathode, a binder and a conductiveagent, which are typically used in the art, may be used in the anode. Ananode active material and the above additives are mixed and stirred toprepare an anode active material composition. Then, a current collectoris coated therewith and pressed to prepare the anode.

The separator is disposed between the cathode and the anode, and a thininsulating film having high ion permeability and mechanical strength maybe used as the separator. Since the separator is known in the art, thedetailed descriptions thereof are omitted in the present specification.

A shape of the lithium secondary battery of the present invention is notparticularly limited, and for example, a cylindrical type using a can, aprismatic type, a pouch type, or a coin type may be used.

The lithium secondary battery according to the present invention may notonly be used in a battery cell that is used as a power source of a smalldevice, but may also be used as a unit cell in a medium and large sizedbattery module including a plurality of battery cells.

Preferred examples of the medium and large sized device may be anelectric vehicle, a hybrid electric vehicle, a plug-in hybrid electricvehicle, or a power storage system, but the medium and large sizeddevice is not limited thereto.

Hereinafter, the present invention will be described in detail,according to specific examples. The invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein. Rather, these example embodiments areprovided so that this description will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art.

EXAMPLES

Hereinafter, the present invention will be described in more detail,according to examples and experimental examples. However, the presentinvention is not limited thereto.

<Preparation of Cathode Active Material>

Example 1

MOOH (m=Ni_(0.78)Mn_(0.11)Co_(0.11)) was used as a mixed transitionmetal precursor, the mixed transition metal precursor and Li₂CO₃ weremixed at a stoichiometric ratio (Li:M=1.00:1), andLiNi_(0.78)Mn_(0.11)CO_(0.11)O₂ was prepared by sintering the mixture ina temperature range of about 800° C. to about 900° C. for 10 hours inair.

LiNi_(0.78)Mn_(0.11)CO_(0.11)O₂ and H₃BO₃ were weighed at a weight ratioof 100:0.17 and mixed with a dry mixer (CYCLOMIX, HOSOKAWA MicronCorporation) to obtain mixed powder. The powder thus obtained was heattreated at 150° C. for 5 hours in an oxygen atmosphere. A cathode activematerial including LiBO₂ and Li₂B₄O₇ in the surface ofLiNi_(0.78)Mn_(0.11)CO_(0.11)O₂ was obtained by the above method. Athickness of the coating layer was 150 nm.

Example 2

A cathode active material was prepared in the same manner as in Example1 except that LiNi_(0.78)Mn_(0.11)CO_(0.11)O₂ and H₃BO₃ were used at aweight ratio of 100:0.34.

In the cathode active material, a thickness of the coating layer was 230nm.

Example 3

A cathode active material was prepared in the same manner as in Example1 except that LiN_(0.78)Mn_(0.11)Co_(0.11)O₂ and H₃BO₃ were used at aweight ratio of 100:0.68.

In the cathode active material, a thickness of the coating layer was 300nm.

Comparative Example 1

A cathode active material was prepared in the same manner as in Example1 except that LiNi_(0.78)Mn_(0.11)Co_(0.11)O₂ and H₃BO₃ were used at aweight ratio of 100:0.09 and a heat treatment was not performed.

In the cathode active material, a thickness of the coating layer was 100nm.

Comparative Example 2

A cathode active material was prepared in the same manner as inComparative Example 1 except that LiNi_(0.78)Mn_(0.11)Co_(0.11)O₂ andH₃BO₃ were used at a weight ratio of 100:0.17.

In the cathode active material, a thickness of the coating layer was 150nm.

Comparative Example 3

A cathode active material was prepared in the same manner as inComparative Example 1 except that LiNi_(0.78)Mn_(0.44)Co_(0.44)O₂ andH₃BO₃ were used at a weight ratio of 100:0.34.

In the cathode active material, a thickness of the coating layer was 160nm.

<Preparation of Lithium Secondary Battery>

Example 4 Cathode Preparation

The cathode active material prepared in Example 1, which included acoating layer containing LiBO₂ and Li₂B₄O₇ in the surface ofLiNi_(0.78)Mn_(0.44)Co_(0.44)O₂, was used.

A cathode mixture slurry was prepared by adding 94 wt % of the cathodeactive material, 3 wt % of carbon black as a conductive agent, and 3 wt% of PVdF as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. Anabout 20 μm thick aluminum (Al) thin film as a cathode collector wascoated with the cathode mixture slurry and dried, and the Al thin filmwas then roll-pressed to prepare a cathode.

Anode Preparation

An anode active material slurry was prepared by mixing 96.3 wt % ofcarbon powder as an anode active material, 1.0 wt % of super-p as aconductive agent, and 1.5 wt % of styrene-butadiene rubber (SBR) and 1.2wt % of carboxymethyl cellulose (CMC) as a binder, and adding themixture to NMP as a solvent. A 10 μm thick copper (Cu) thin film as ananode collector was coated with the anode active material slurry anddried, and the Cu thin film was then roll-pressed to prepare an anode.

Non-Aqueous Electrolyte Solution Preparation

A 1 M LiPF₆ non-aqueous electrolyte solution was prepared by addingLiPF₆ to a non-aqueous electrolyte solvent that was prepared by mixingethylene carbonate and diethyl carbonate, as an electrolyte, at a volumeratio of 30:70.

Lithium Secondary Battery Preparation

A mixed separator of polyethylene and polypropylene was disposed betweenthe cathode and anode thus prepared, and a polymer type battery was thenprepared by a typical method. Then, the preparation of each lithiumsecondary battery was completed by injecting the prepared non-aqueouselectrolyte solution.

Examples 5 and 6

Lithium secondary batteries were prepared in the same manner as inExample 4 except that the cathode active materials prepared in Examples2 and 3 were respectively used.

Comparative Examples 4 to 6

Lithium secondary batteries were prepared in the same manner as inExample 4 except that the cathode active materials prepared inComparative Examples 1 to 3 were respectively used.

Experimental Example 1 Inductively Coupled Plasma (ICP) Mass Analysis

In order to investigate the amounts of elemental B included in thecoating layers of the cathode active materials prepared in Examples 1 to3 and Comparative Examples 1 to 3, the coating layers were analyzed byinductively coupled plasma-atomic emission spectroscopy (ICP-AES).

Specifically, 0.1 g of each of the cathode active materials prepared inExamples 1 to 3 and Comparative Examples 1 to 3 was sampled, 2 ml ofdistilled water and 3 ml of concentrated nitric acid were added thereto,and each sample was dissolved after closing a lid. After the sample wascompletely dissolved, the solution was diluted by adding 50 ml ofultrapure water thereto. Thereafter, the diluted solution was againdiluted in 10 times and then analyzed with an inductively coupledplasma-atomic emission spectrometer (ICP-AES). The ICP-AES (ICP 5300DV,PerkinElmer Inc.) was operated under the following conditions: ForwardPower 1,300 W; Torch Height 15 mm, Plasma Gas Flow Rate 15.00 L/min;Sample Gas Flow Rate 0.8 L/min; Auxiliary Gas Flow Rate 0.20 L/min; andPumping Speed 1.5 ml/min. As a result, the amounts of elemental Bincluded in the coating layers of the cathode active materials preparedin Examples 1 to 3 and Comparative Examples 1 to 3 are presented inTable 1 below.

TABLE 1 B content (ICP) Amount of H₃BO₃ B (experimental B (theoreticalExamples used (wt %) value, ppm) value, ppm) Example 1 0.17 275 290Example 2 0.34 575 585 Example 3 0.68 1,110 1,170 Comparative 0.09 145145 Example 1 Comparative 0.17 290 290 Example 2 Comparative 0.34 290585 Example 3

As illustrated in Table 1, there was a difference between the amounts ofthe elemental B included in the examples subjected to the heat treatmentat 150° C. and the comparative examples not subjected to a heattreatment even if the same amount of H₃BO₃ was used.

Specifically, with respect to Examples 1 to 3 subjected to the heattreatment at 150° C., i.e., near the melting point of H₃BO₃, the amountof the elemental B included in the cathode active material was alsoincreased from 275 ppm to 1,110 ppm similar to the theoretical value asthe amount of the H₃BO₃ used was increased from 0.17 wt % to 0.68 wt %.

However, with respect to Comparative Examples 1 to 3 not subjected to aheat treatment, as a result of changing the amount of the H₃BO₃ used to0.09 wt %, 0.17 wt %, and 0.34 wt %, the amount of the elemental Bincluded in the cathode active material was increased similar to thetheoretical value in the case that the amount of the H₃BO₃ used was 0.09wt % and 0.17 wt %. However, in the case in which the amount of theH₃BO₃ used was 0.34%, the amount of the elemental B was the same as thatof the case of using 0.17 wt %.

In the case that the heat treatment was not performed as in thecomparative examples, it may be understood that the amount of theelemental B included in the cathode active material was not increasedwhen adding H₃BO₃ in a predetermined amount or more.

In contrast, in the case that the heat treatment was performed as inExamples 1 to 3, it may be understood that the amount of the elemental Bwas detected from the cathode active material by the amount of the H₃BO₃used.

Experimental Example 2 pH Titration Experiment to Investigate the Amountof Lithium Impurities

In order to investigate the amounts of lithium impurities in the cathodeactive materials prepared in Example 1 and Comparative Examples 1 and 2,pH titrations were performed, and the results thereof are presented inFIG. 2. Metrohm 794 was used as a pH meter and pH values were recordedwhile titrating in 0.02 ml increments.

FIG. 2 is a graph comparing the amounts of lithium impurities reduced inrespective cathode active materials of Example 1 and ComparativeExamples 1 and 2.

That is, as a result of comparing the amounts of 0.1 M HCl for 10 g ofthe respective cathode active materials, when comparing the amounts ofhydrochloric acid used for hydrochloric acid titration with reference toFIG. 2, Example 1 was about 11.6 ml, Comparative Example 1 was 15 ml,and Comparative Example 2 was about 14.2 ml. Thus, it may be understoodthat Example 1 was decreased by about 20% or more in comparison toComparative Examples 1 and 2.

Experimental Example 3 Output Characteristics After High-TemperatureStorage and Resistance Increase Rate Evaluation Tests

FIG. 3 is a graph illustrating the results of measuring capacitycharacteristics after high-temperature storage (60° C.) of the lithiumsecondary batteries of Example 5 and Comparative Example 4 according toExperimental Example 3.

The lithium secondary batteries of Example 5 (cathode active material ofExample 2) and Comparative Example 4 (cathode active material ofComparative Example 1) were stored at 60° C. and then charged at 1 C to4.15 V/30 mA under a constant current/constant voltage (CC/CV)condition. Then, the lithium secondary batteries were discharged at aconstant current (CC) of 1 C to a voltage of 2.5 V to measure dischargecapacities. The results thereof are presented in FIG. 3.

Referring to FIG. 3, a slope of a capacity retention ratio of thelithium secondary battery of Example 5 of the present invention wasslower than that of the lithium secondary battery of Comparative Example4 to a storage time of 4 weeks. Specifically, it may be understood thatthe capacity retention ratio of the lithium secondary battery of Example5 was increased by about 3% at a storage time of 4 weeks in comparisonto that of the lithium secondary battery of Comparative Example 4.

Also, it may be understood that a resistance increase rate of thelithium secondary battery of Example 5 was decreased by about 25% incomparison to that of the lithium secondary battery of ComparativeExample 4. It may be understood that the decrease in the resistanceincrease rate to about 25% may also affect output characteristics. Thatis, it may be understood that the lithium secondary battery of Example 5may exhibit excellent output characteristics due to the decrease in theresistance increase rate.

Experimental Example 4 High-Temperature (45° C.) Cycle Characteristicsand Resistance Increase Rate Evaluation Tests

The lithium secondary batteries of Example 5 and Comparative Example 4were charged at 1 C to 4.15 V/30 mA at 45° C. under a constantcurrent/constant voltage (CC/CV) condition and then discharged at aconstant current (CC) of 2 C to a voltage of 2.5 V to measure dischargecapacities. The charge and discharge were repeated 1 to 400 cycles andthe measured discharge capacities are presented in FIG. 4.

Referring to FIG. 4, the lithium secondary battery of Example 5 of thepresent invention exhibited the slope of the capacity retention ratiosimilar to that of Comparative Example 4 to the 400^(th) cycle.

However, it may be understood that the resistance increase rate of thelithium secondary battery of Example 5 was decreased by about 7% at the400^(th) cycle in comparison to that of the lithium secondary battery ofComparative Example 4. It may be understood that the decrease in theresistance increase rate may also affect output characteristics. Thatis, it may be understood that the lithium secondary battery of Example 5may exhibit excellent output characteristics due to the decrease in theresistance increase rate.

1. A method of preparing a cathode active material, the methodcomprising coating a surface of a lithium transition metal oxide with alithium boron oxide by dry mixing the lithium transition metal oxide anda boron-containing compound and performing a heat treatment.
 2. Themethod of claim 1, wherein the heat treatment is performed in atemperature range of 130° C. to 300° C.
 3. The method of claim 2,wherein the heat treatment is performed in a temperature range of 130°C. to 200° C.
 4. The method of claim 2, wherein the boron-containingcompound is transformed into a lithium boron oxide through a reactionwith at least a portion of lithium impurities in the lithium transitionmetal oxide by the heat treatment.
 5. The method of claim 2, wherein aportion of elemental boron (B) of the lithium boron oxide is doped intothe lithium transition metal oxide by the heat treatment, and an amountof the B has a concentration gradient gradually decreasing from thesurface of the lithium transition metal oxide to inside thereof.
 6. Themethod of claim 4, wherein the lithium impurities comprise LiOH, Li₂CO₃,or a mixture thereof.
 7. The method of claim 1, wherein the mixing isperformed by a mortar grinder mixing method or a mechanical millingmethod.
 8. The method of claim 7, wherein the mixing by the mechanicalmilling method is performed by using a roll mill, ball mill, high energyball mill, planetary mill, stirred ball mill, vibrating mill, or jetmill.
 9. The method of claim 1, wherein the boron-containing compoundcomprises any one selected from the group consisting of H₃BO₃, B₂O₃,C₆H₅B(OH)₂, (C₆H₅O)₃B, [CH₃(CH₂)₃O]₃B, C₁₃H₁₉BO₃, C₃H₉B₃O₆, and(C₃H₇O)₃B, or a mixture of two or more thereof.
 10. The method of claim1, wherein the boron-containing compound is used in an amount of 0.05 wt% to 1 wt % based on a total weight of the lithium transition metaloxide.
 11. The method of claim 1, wherein the lithium boron oxide isLiBO₂, Li₂B₄O₇, or a mixture thereof.
 12. The method of claim 1, whereinthe lithium transition metal oxide is represented by Chemical Formula 1:Li_(1+a)[Ni_(x)Mn_(y)Co_(z)M_(v)]O_(2-c)A_(c)  <Chemical Formula 1>where M is any one selected from the group consisting of aluminum (Al),zirconium (Zr), zinc (Zn), titanium (Ti), magnesium (Mg), gallium (Ga),and indium (In), or two or more elements thereof; A is at least oneselected from the group consisting of phosphorus (P), fluorine (F),sulfur (S), and nitrogen (N), and 0≦x≦1.0, 0≦y<0.6, 0≦z<0.6, 0≦v≦0.1,0≦a<0.3, 0≦c≦0.2, and a+x+y+z+v=1.
 13. A cathode active materialcomprising: a lithium transition metal oxide; and a coating layerincluding a lithium boron oxide on a surface of the lithium transitionmetal oxide.
 14. The cathode active material of claim 13, wherein thecoating layer comprises elemental boron (B) in an amount of 100 ppm to2,000 ppm.
 15. The cathode active material of claim 13, wherein thecoating layer comprises elemental B in an amount of 250 ppm to 1,100ppm.
 16. The cathode active material of claim 13, wherein a thickness ofthe coating layer is in a range of 10 nm to 1,000 nm.
 17. The cathodeactive material of claim 13, wherein a portion of elemental B of thelithium boron oxide is further included in the lithium transition metaloxide.
 18. The cathode active material of claim 17, wherein the lithiumtransition metal oxide is represented by Chemical Formula 3, and, inChemical Formula 3, w has a concentration gradient gradually decreasingfrom the surface of the lithium transition metal oxide to insidethereof:Li_(1+a)[Ni_(x)Mn_(y)Co_(z)B_(w)M_(v)]O_(2-c)A_(c)  <Chemical Formula 3>where M is any one selected from the group consisting of aluminum (Al),zirconium (Zr), zinc (Zn), titanium (Ti), magnesium (Mg), gallium (Ga),and indium (In), or two or more elements thereof; A is at least oneselected from the group consisting of phosphorus (P), fluorine (F),sulfur (S), and nitrogen (N), and 0≦x≦1.0, 0≦y<0.6, 0≦z<0.6, 0≦v≦0.1,0≦a<0.3, 0≦c≦0.2, a+x+y+z+v=1, and 0≦w≦0.1.
 19. The cathode activematerial of claim 13, wherein the cathode active material compriseslithium impurities in an amount of less than 0.3 wt % based on a totalweight of the cathode active material.
 20. The cathode active materialof claim 13, wherein the lithium boron oxide is LiBO₂, Li₂B₄O₇, or amixture thereof.
 21. A cathode comprising the cathode active material ofclaim
 13. 22. A lithium secondary battery comprising the cathode ofclaim 21.